The invention relates to a process for the preparation of polymers stabilized with protective colloids and water-redispersible powders or aqueous dispersions having a reduced content of volatile components prepared therefrom.
A growing ecological consciousness has resulted in more strenuous requirements on the market with respect to freedom from residual monomers and solvents in many polymer products. These requirements together are characterized by the term xe2x80x9cVOC-freexe2x80x9d (VOC=volatile organic compound). While VOC-free polymer dispersions are now common, this is not yet the case with redispersion powders. Possible explanations for this are that the volatile components in solid products are more difficult to remove than in liquid products, and that the volatile components become chemically bound when redispersion powders are used in many aqueous formulations. For example, residual vinyl acetate is hydrolyzed directly to calcium acetate and acetaldehyde in applications in cement media the main field of use of redispersion powders, the acetaldehyde immediately undergoing aldol condensations. The same occurs with other esters, such as methyl or ethyl acetate, and with acetone (aldol condensation).
However, very recent investigations have shown that considerable contamination by volatile components can nevertheless occur; for example, acetaldehyde is liberated when vinyl acetate-based redispersion powders are used in levelling compounds or floor toppings. The acetaldehyde is present in the powder or is formed by hydrolysis from residual vinyl acetate present in the powder.
A number of processes for removing volatile components from polymers are known from the prior art. A distinction may be made between chemical and physical deodorizing processes. The chemical processes are distinguished by the addition of substances which react with the residual monomer and thus reduce their content. The physical processes are substantially based on distillation or stripping phenomena with steam or inert gases such as nitrogen as entraining agents. In addition, combinations of these two processes have been described.
DE-A 19741185 describes the reduction of residual monomer content of polymer dispersions by postpolymerization with a special redox system comprising carboxylic acid and peroxide compounds. A further chemical process for removing residual monomers from polyvinyl ester dispersions is disclosed in EP-B 505959. In the latter reference, the polyvinyl ester dispersion is subjected to hydrolysis at weakly alkaline pH followed by a subsequent oxidative treatment. DE-A 19741189 describes a chemical process for removing residual monomers, in which a nucleophilic agent is metered into a specially dimensioned reactor in a defined mixing time.
The physical removal of volatile residues from polymer dispersions by passing in steam is disclosed in DE-A 19745580, and removal by means of inert gas, for example air or nitrogen, is disclosed in DE-A 4118526. A combined process for removing volatile components, in which first a postpolymerization with redox initiator and then an inert gas treatment are carried out, is disclosed in DE-A 19828183. In the process from EP-A 650977, the residual monomers are first removed by postpolymerization and then by means of steam stripping. EP-A 465964 describes a process for removing volatile components from emulsion polymers, these first being spray-dried and the powder then being aftertreated with inert gas.
It is known that chemical measures reduce residual monomer content by introducing other volatile components, such as tert-butanol and acetone, into the dispersion. It is, however, senseless and uneconomical to carry out these measures to achieve extremely low residual monomer content, since at the same time the content of other volatile components increases disproportionately. On the other hand, it is known that physical deodorization can result in damage to the dispersion, ranging from the presence of specks to coagulum. Exclusive physical deodorization is therefore also not expedient since the products are likely to be damaged owing to high levels of contamination, even with processes according to the latest state of the art. The considerable amounts of condensate contaminated with organic components, about 10 to 20%, based on the dispersion used, which, depending on the composition, have to be disposed of by expensive procedures, constitute further considerable disadvantages of physical deodorization.
An additional problem in the preparation of redispersion powders is the use of methanol-containing polyvinyl alcohol as a protective colloid during spraying. This protective colloid is added to the dispersion only after chemical and physical deodorization, so that the methanol content of the dispersion is high as a result.
It would be desirable to provide an economical process for the preparation of polymers stabilized with protective colloids in the form of water-redispersible powders or aqueous dispersions which have a reduced content of volatile components. It would be further desirable to provide such a process which avoids the disadvantages of the prior art.
The invention relates to a process for the preparation of polymers stabilized with protective colloids in the form of water-redispersible powders or aqueous dispersions which have a reduced content of volatile components, this process comprising
a) polymerizing by emulsion or suspension polymerization, one or more monomers from the group consisting of vinyl esters, (meth)acrylates, vinyl aromatics, olefins, 1,3-dienes and vinyl halides and, if required, further monomers copolymerizable therewith,
b) aftertreating the polymer dispersion thus obtained by means of postpolymerization and/or distillation, and introduction of steam or inert gas to a residual content of volatile, nonaqueous components of  less than 2000 ppm, and then
c) spray drying the aftertreated polymer dispersion to a residual content of volatile, nonaqueous components of  less than 400 ppm, the dispersion being adjusted to a solids content of  less than 45% by weight before spraying, the spray drying carried out with air at an inlet temperature of  greater than 120xc2x0 C, and optionally,
d) redispersing the resulting powder in water.
Volatile components (VOC) are understood as meaning nonaqueous compounds, such as residual monomers, decomposition products of monomer and initiator, impurities in polymerization assistants and the like; for example, residual monomers such as vinyl acetate, alkyl (meth)acrylate and styrene; decomposition products or impurities such as acetaldehyde, methanol, ethanol, tert-butanol, acetone, methyl acetate, and ethyl acetate.
Preferred vinyl esters are those of carboxylic acids having 1 to 12 C atoms. Vinyl acetate, vinyl propionate, vinyl butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl acetate, vinyl pivalate and vinyl esters of xcex1-branched monocarboxylic acids having 9 to 11 C atoms, for example VeoVa9R or VeoVa10R (trade names of Shell), are more preferred. Vinyl acetate is particularly preferred.
Suitable monomers from the group consisting of acrylates or methacrylates are esters of straight-chain or branched alcohols having 1 to 15 carbon atoms. Preferred methacrylates or acrylates are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate and 2-ethylhexyl acrylate. Methyl acrylate, methyl methacrylate, n-butyl acrylate, tert-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred.
Preferred vinyl aromatics are styrene, methylstyrene and vinyltoluene. A preferred vinyl halide is vinyl chloride. The preferred olefins are ethylene and propylene and the preferred dienes are 1,3-butadiene and isoprene.
If required, 0.1 to 5% by weight, based on the total weight of the monomer mixture, of auxiliary monomers may also be copolymerized. Preferably, 0.5 to 2.5% by weight of auxiliary monomers are used. Examples of auxiliary monomers are ethylenically unsaturated mono- and dicarboxylic acids, preferably acrylic acid, methacrylic acid, fumaric acid and maleic acid; ethylenically unsaturated carboxamides and carbonitriles, preferably acrylamide and acrylonitrile; mono- and diesters of fumaric acid and maleic acid, such as the diethyl and diisopropyl esters, and maleic anhydride and ethylenically unsaturated sulfonic acids and their salts, preferably vinyl sulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid. Further examples are precrosslinking comonomers, such as polyethylenically unsaturated comonomers such as divinyl adipate, diallyl maleate, allyl methacrylate or triallyl cyanurate, and postcrosslinking comonomers, for example acrylamidoglycolic acid (AGA), methyl acrylamidoglycolic acid methyl ester (MAGME), N-methylolacrylamide (NMA), N-methylolmethacrylamide, -methylolallylcarbamate, alkyl ethers or esters of N-methylolacrylamide, -methylolmethacrylamide and N-methylolallylcarbamate, such as the isobutoxy ethers. Comonomers having epoxide functional groups such as glycidyl methacrylate and glycidyl acrylate, are also suitable. Further examples are comonomers having silicon functional groups, such as acryloyloxypropyltri(alkoxy)- and methacryloyloxypropyltri(alkoxy)silanes, vinyltrialkoxysilanes and vinylmethyldialkoxysilanes, it being possible, for example, for ethoxy and ethoxypropylene glycol ether radicals to be present as alkoxy groups. Monomers having hydroxyl or CO groups may also be mentioned, for example hydroxyalkyl methacrylates and acrylates such as hydroxyethyl, hydroxypropyl or hydroxybutyl acrylate or methacrylate, and compounds such as diacetoneacrylamide and acetylacetoxyethyl acrylate or methacrylate.
Monomers or monomer mixtures which contain one or more monomers from the group consisting of vinyl acetate, vinyl esters of xcex1-branched monocarboxylic acids having 9 to 11 C atoms, vinyl chloride, ethylene, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate and styrene are particularly preferred. Most preferred are mixtures comprising vinyl acetate and ethylene; mixtures comprising vinyl acetate, ethylene and a vinylester of xcex1-branched monocarboxylic acids having 9 to 11 C atoms; mixtures comprising n-butyl acrylate and 2-ethylhexyl acrylate and/or methyl methacrylate; mixtures comprising styrene and one or more monomers from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; mixtures comprising vinyl acetate and one or more monomers from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate and optionally ethylene; and mixtures comprising 1,3-butadiene and styrene and/or methyl methacrylate and optionally further acrylates. These mixtures may also optionally contain one or more of the abovementioned auxiliary monomers, and each of these preferred mixtures may be free of auxiliary monomers or other monomers other than those specified, if desired.
The choice of monomers and/or the choice of the amounts by weight of comonomers is made in such a way that in general, a glass transition temperature (xe2x80x9cTgxe2x80x9d) of xe2x88x9250xc2x0 C. to +50xc2x0 C., preferably xe2x88x9230xc2x0 C. to +40xc2x0 C., results. The Tg of the polymer can be determined conventionally by means of differential scanning calorimetry (DSC). The Tg can also be calculated approximately beforehand using the Fox equation. According to T. G. Fox, BULL. AM. PHYSICS SOC. 1, 3, page 123 (1956), the following is applicable: 1/Tg=x1/Tg1+x2/Tg2+ . . . +xn/Tgn, where xn is the mass fraction (% by weight/100) of the monomer n and Tgn is the glass transition temperature in Kelvin of the homopolymer of the monomer n. Tg values for homopolymers appear in the POLYMER HANDBOOK, 2nd Edition, J. Wiley and Sons, New York (1975).
The polymers are prepared in a conventional manner by a emulsion polymerization process or by a suspension polymerization process, preferably by the emulsion polymerization process, the polymerization temperature being in general from 40xc2x0 C. to 100xc2x0 C., preferably from 60xc2x0 C. to 90xc2x0 C. In the copolymerization of gaseous comonomers, such as ethylene, 1-3-butadiene or vinyl chloride, superatmospheric pressure, in general from 5 bar to 100 bar, may be employed.
The polymerization is initiated with water-soluble or monomer-soluble initiators or redox initiator combinations customarily used for emulsion polymerization or suspension polymerization. Examples of water-soluble initiators are the sodium, potassium and ammonium salts of peroxodisulfuric acid, hydrogen peroxide, tert-butyl peroxide, tert-butyl hydroperoxide, potassium peroxodiphosphate, tert-butyl peroxypivalate, cumyl hydroperoxide, isopropylbenzyl monohydroperoxide and azobisisobutyronitrile. Examples of monomer-soluble initiators are diacetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate and dibenzoyl peroxide. The initiators are generally used in an amount of 0.01 to 0.5% by weight, based on the total weight of the monomers. Combinations of said initiators in combination with reducing agent(s) May be used as redox initiators. Suitable reducing agents are the sulfites and bisulfites of alkali metals and of ammonium, for example sodium sulfite, derivatives of sulfoxylic acid such as zinc or alkali metal formaldehyde sulfoxylates, for example sodium hydroxymethanesulfonate, and ascorbic acid. The amount of reducing agent is preferably 0.01 to 0.5% by weight, based on the total weight of the monomers.
Molecular weight regulating substances (chain transfer agents) may be used for controlling the molecular weight during polymerization. If regulators are used, they are usually employed in amounts of 0.01 to 5.0% by weight based on the monomers to be polymerized, and may be metered separately, or may be premixed with one or more reaction components before being metered. Examples of such substances are n-dodecyl mercaptan, tert-dodecyl mercaptan, mercaptopropionic acid, methyl mercaptopropionate, isopropanol and acetaldehyde. Preferably, no regulating substances are used.
Suitable protective colloids are partially hydrolyzed polyvinyl alcohols; polyvinylpyrrolidones; polyvinyl acetals; polysaccharides in water-soluble form, such as starches (amylose and amylopectin), and celluloses and their derivatives such as the carboxymethyl, methyl, hydroxyethyl and hydroxypropyl derivatives; proteins such as casein or caseinates, soybean protein and gelatin; ligninsulfonates; synthetic polymers such as (meth)acrylic acid, copolymers of (meth)acrylates with comonomer units having carboxyl functional groups, poly(meth)acrylamide, polyvinylsulfonic acids and their water-soluble copolymers; melamine formaldehyde sulfonates, naphthalene formaldehyde sulfonates and styrene maleic acid and vinyl ether/maleic acid copolymers.
Partially hydrolyzed or completely hydrolyzed polyvinyl alcohols are preferred. Partially hydrolyzed polyvinyl alcohols having a degree of hydrolysis of 80 to 95 mol % and a Hxc3x6ppler viscosity, in 4% strength aqueous solution, of 1 to 30 mPaxc2x7s (Hxc3x6ppler method at 20xc2x0 C., DIN 53015) are particularly preferred. Partially hydrolyzed, hydrophobically modified polyvinyl alcohols having a degree of hydrolysis of 80 to 95% mol and a Hxc3x6ppler viscosity, in 4% strength aqueous solution, of 1 to 30 mPaxc2x7s are also suitable. Examples of these are partially hydrolyzed copolymers of vinyl acetate with hydrophobic comonomers such as isopropenyl acetate, vinyl pivalate, vinyl ethylhexanoate, vinyl esters of saturated, alpha-branched monocarboxylic acids having 5 or 9 to 11 C atoms, dialkyl maleates and dialkyl fumarates such as diisopropyl maleate and diisopropyl fumarate; vinyl chloride; vinyl alkyl ethers such as vinyl butyl ether; and olefins, such as ethene and decene. The amount of the hydrophobic unit is preferably 0.1 to 10% by weight, based on the total weight of the partially hydrolyzed polyvinyl alcohol. Mixtures of these polyvinyl alcohols may also be used.
Further suitable polyvinyl alcohols are partially hydrolyzed polyvinyl alcohols which have been rendered hydrophobic and are obtained by polymer-analogous reaction, for example acetalation of vinyl alcohol moieties with C1- to C4-aldehydes such as butyraldehyde. The amount of the hydrophobic units is preferably 0.1 to 10% by weight, based on the total weight of the partially hydrolyzed polyvinyl acetate. The degree of hydrolysis is 80 to 95 mol %, preferably 85 to 94 mol %, and the Hxc3x6ppler viscosity (DIN 53015, Hxc3x6ppler method, 4% strength aqueous solution) is preferably from 1 to 30 mPaxc2x7s, more preferably 2 to 25 mPaxc2x7s.
Most preferred are polyvinyl alcohols having a degree of hydrolysis of 85 to 94 mol % and a Hxc3x6ppler viscosity, in 4% strength aqueous solution, of 3 to 15 mPaxc2x7s (Hxc3x6ppler method at 20xc2x0 C., DIN 53015). The protective colloids are obtainable by processes known to those skilled in the art.
In the process according to the invention, polymerization is preferably effected without the addition of emulsifiers. In certain cases, it may be advantageous additionally to use small amounts of emulsifiers, for example 1 to 5% by weight, based on the total amount of monomers. Suitable emulsifiers are anionic, cationic and nonionic emulsifiers, for example anionic surfactants, such as alkylsulfates having a chain length of 8 to 18 C atoms, alkyl or alkylaryl ether sulfates having 8 to 18 C atoms in the hydrophobic radical and up to 40 ethylene oxide or propylene oxide units, alkyl- or alkylarylsulfonates having 8 to 18 C atoms, esters and monoesters of sulfosuccinic acid with monohydric alcohols or alkylphenols, or nonionic surfactants such as alkyl polyglycol ethers or alkylaryl polyglycol ethers having 8 to 40 ethylene oxide units.
The protective colloids are generally added during the polymerization in a total amount of 1 to 20% by weight, based on the total weight of the monomers. The protective colloid may be either completely introduced initially, or partly introduced initially and partly metered in during the polymerization. Preferably, at least 5% by weight of the protective colloid is initially introduced, and most preferably all of the protective colloid is initially introduced.
The monomers can be initially introduced in total, metered in in total or initially introduced in parts and the remainder metered in after initiation of the polymerization. In a preferred procedure, 50 to 100% by weight, based on the total weight of the monomers, are initially introduced and the remainder is metered in. The metering can be carried out separately or some or all of the components to be metered can be preemulsified before being metered. The metering may be continuous or in increments.
In the preparation of low-VOC redispersion powders based on polymers stabilized with polyvinyl alcohol as the protective colloid, the methanol content of the polyvinyl alcohol used is of considerable importance. As a result of their preparation, polyvinyl alcohols contain considerable amounts of methanol. Methanol may be introduced along with the polyvinyl alcohol introduced into the dispersion in which the polyvinyl alcohol acts as a protective colloid during polymerization, or may be introduced with the polyvinyl alcohol which is added as a protective colloid before spray drying, introducing methanol into the system at this point. Being inert with respect to the other constituents, methanol can be separated only by physical methods such as distillation. Advantageously, this separation occurs during the preparation of the polyvinyl alcohol solution itself prior to its addition to the polymerization batch, or the separation of methanol takes place from the prepared dispersion, prior to spraying. Industrially produced products are sold with about 3% by weight of methanol. In the context of the present invention, polyvinyl alcohol having a residual content of  less than 2% by weight is preferably used, more preferably polyvinyl alcohol containing  less than 1% by weight of methanol.
As an alternative to removing methanol during the preparation of the polyvinyl alcohol solution, the methanol can, of course, be separated by steam distillation of the dispersion. However, it should be ensured that the distillation is carried out at the correct point in the process chain, i.e. ideally only when the polyvinyl alcohol, which serves as a protective colloid for spraying, has already been added. However, this generally cannot be realized in practice since the physical deodorization is effected in plants other than the plant used for the spray drying, so that the removal of methanol during the dissolution step is preferable. A continuous distillation over a column, upstream of the spray drying, can be considered as a further variant, which however requires considerable additional costs in terms of process engineering.
After the end of the polymerization, the amount of volatile, nonaqueous components in the dispersion obtained is reduced to xe2x89xa62000 ppm, preferably xe2x89xa61000 ppm, most preferably xe2x89xa6500 ppm. This can be effected by means of postpolymerization, by distillation, or by means of inert gases such as steam, or by a combination of these measures.
Redox initiator systems, for example those comprising the abovementioned oxidizing agents and reducing agents, may be used for a postpolymerization. In general, an initiator system which differs from that used for the main polymerization is employed for the postpolymerization. Redox combinations of hydrogen peroxide, sodium or potassium peroxide or tert-butyl hydroperoxide with sodium sulfite, alkali metal formaldehyde sulfoxylates or ascorbic acid, are preferred for the postpolymerization. The postpolymerization is carried out in general at temperatures of from 30xc2x0 C. to from 60xc2x0 C. and over a period of 0.5 to 3 hours. The components of the redox system can be added in portions or may be metered in continuously. The amount of oxidizing agent and reducing agent is in general 0.01 to 0.4% by weight, based on total monomer weight.
The volatile components may also be removed by means of distillation, preferably under reduced pressure, or while passing inert entraining gases such as air, nitrogen or steam through or over the dispersion. In a preferred procedure for aftertreatment with steam, not more than 5 to 10% by weight of condensate, based on the weight of dispersion, is introduced. In general, the aftertreatment is carried out at 50xc2x0 C. to 80xc2x0 C., under a reduced pressure of 200 to 500 mbar and over a period of one to three hours. In the first step for removing the volatile components, the postpolymerization is particularly preferably combined with an inert gas treatment.
For the preparation of the water-redispersible polymer powders the aqueous dispersions are spray-dried after the addition of protective colloids as spraying assistants. The solids content of the dispersion is adjusted to values below 45% by weight, preferably 30 to 40% by weight, before the spray drying. The spray drying is carried out in conventional spray drying units, it being possible to effect atomization by means of airless high-pressure nozzles, binary nozzles, multi-medium nozzles, or by means of a rotating disk. What is important is that the inlet temperature of the gas stream is  greater than 120xc2x0 C. By means of the spray drying, the content of nonaqueous, volatile components in the powder is reduced to values of  less than 400 ppm, preferably in the range of 1 to 250 ppm, and in particular from 1 to 50 ppm. The gas employed in spray drying may be any gas inert to the product, but is preferably air.
As a rule, the spraying assistant is used in a total amount of 3 to 30% by weight, based on the polymeric components of the dispersion. This means that the total amount of the protective colloid before the drying process should be at least 3 to 30% by weight, based on the polymer fraction, and preferably from 5 to 20% by weight.
Suitable spraying assistants are partially hydrolyzed polyvinyl alcohols; polyvinylpyrrolidones; polysaccharides in water-soluble form, such as starches (amylose and amylopectin), celluloses and their carboxymethyl, methyl, hydroxyethyl and hydroxypropyl derivatives; proteins, such as casein or caseinate, soybean protein, gelatin; ligninsulfonates; synthetic polymers, such as poly(meth)acrylic acid, copolymers of (meth)acrylates with comonomer units having carboxyl functional groups, poly(meth)acrylamide, polyvinylsulfonic acids and their water-soluble copolymers; melamine formaldehyde sulfonates, naphthalene formaldehyde sulfonates and styrene/maleic acid and vinyl ether/maleic acid copolymers.
Polyvinyl alcohols having a degree of hydrolysis of from 80 to 95 mol % and a Hxc3x6ppler viscosity, in 4% strength aqueous solution, of 1 to 30 mPaxc2x7s (Hxc3x6ppler method at 20xc2x0 C., DIN 53015) are preferred. Most preferably, no protective colloids other than polyvinyl alcohols are used as spraying assistants.
A content of up to 1.5% by weight of antifoam, based on the weight of the base polymers, has proven advantageous during spraying. In order to increase the storability by improving the blocking stability, in particular in the case of powders having a low glass transition temperature, the powder obtained can be treated with an anticaking agent, preferably in an amount of up to 30% by weight, based on the total weight of polymeric components. Examples of anticaking agents are calcium carbonate, magnesium carbonate, talc, gypsum, silica, kaolins, and silicates, preferably having particle sizes in the range from 10 nm to 10 xcexcm.
In order to improve the performance characteristics, further additives may be introduced during the spraying. Further components of dispersion powder compositions, which are present in preferred embodiments, are, for example, pigments, fillers, foam stabilizers and water repellents.
Aqueous polymer dispersions having a reduced content of volatile, nonaqueous components are obtained by redispersion of the spray-dried powders with water. Usually, solids contents of 50 to 60% by weight are established. In the preparation of a 50% strength redispersion, the proportion of volatile components in ppm is reduced by half again compared with the powder.
The aqueous polymer dispersions and the water-redispersible polymer powders stabilized with the protective colloids can be used in the applications typical for them, for example, in chemical products for the building industry, if necessary in combination with hydraulically setting binders such as cements (Portland cement, high-alumina cement, trass cement, slag cement, magnesia cement and phosphate cement), gypsum or waterglass, for the production of construction adhesives, in particular for preparing tile adhesives and heat-insulating adhesives, renders, filling compounds, floor filling compounds, levelling compounds, sealing slurries, joint mortars and paints, and also as binders for coating materials and adhesives or as coating materials and binders for textiles, fibers, wood and paper.
The examples below serve for further illustrating the invention. Example 1 sets forth the general method for the preparation of the dispersion and of the powder.